Clay minerals, also known as hydrous layer silicates, are part of a larger family of minerals called phyllosilicates or layer silicates. A unit layer of a given clay mineral is typically composed of a two dimensional arrangement of tetrahedral and octahedral sheets, each with specific elemental composition. The tetrahedral sheet may have composition of T.sub.2 O.sub.5 where T, the tetrahedral cation, is Si, Al and/or Fe, and the octahedral sheet commonly contains cations such as Al, Fe and Mg. A tetrahedral sheet containing only Si.sup.4+ cations is electrically neutral, as are Mg-octahedral sheets and Al octahedral sheets with one third vacant sites. Both tetrahedral and octahedral sheets can have cationic substitutions resulting in a net negative charge which is balanced by interlayer cations. The type of sheets in a unit layer, degree of substitutions, and stacking of layers vary greatly, and determine the type and economic usefulness of a given clay mineral.
Clay minerals can be broadly classified into 2:1 and 1:1 type on the basis of type of sheets in a unit layer. The 2:1 type clay minerals are composed of one octahedral sheet sandwiched between two tetrahedral sheets (FIG. 1). The upper tetrahedral sheet is inverted so that apical oxygens point down and are shared by the octahedral sheet. When all the sites in the tetrahedral sheet are occupied by Si and all sites in the octahedral sheet are occupied by Mg or two-thirds of the sites are occupied by Al, the resulting layers are electrically neutral. In nature, these conditions are exemplified by talc and pyrophyllite, respectively. Electrically neutral layers of these minerals are coupled by weak dipolar and van der Waals forces. There is no driving force for attraction of interlayer cations or intercalating compounds. Therefore, these minerals have little use as exchangers or adsorbents.
In contrast to talc and pyrophyllite, layers in mica bear a net negative charge of 1e per Si.sub.8 O.sub.20 unit due to excessive cationic substitutions in the tetrahedral sheet. The resultant net negative charge is balanced by interlayer K.sup.+, which is coordinated to the hexagonal array of oxygen atoms on either side of the interlayer space. The interlayer K strongly binds the layers and resist intercalation and exchange processes for these minerals.
While pyrophyllite-talc group and mica group minerals represent the extreme cases with either no or excessive charge, the smectite group represent 2:1 clay minerals with an intermediate level of charge, usually ranging from 0.2 to 0.6 e per Si.sub.8 O.sub.20. Whilst the net negative charge on smectites is sufficient to hold exchangeable cations and a provide driving force for intercalation, it is not too excessive and localised, as in micas, to resist swelling of layers. The interlayer charge in smectites is usually balanced by intercalated alkaline earth or alkali metals which can be readily exchanged with desired cations.
Some smectites (such as beidellite and nontronite) derive their charge from substitutions in tetrahedral sheets, while others derive their charge from substitutions in octahedral sheets. The layer charge in tetrahedrally charged smectites is relatively localised, which results in greater three dimensional order and -poorer intercalation, swelling and exchange properties. In the octahedrally charged smectites, the layer charge is uniformly distributed in the oxygen framework due to the octahedral sheet being sandwiched between tetrahedral sheets. As a result, the adjoining layers are randomly stacked with little three dimensional order. These octahedrally charged smectites, called montmorillonites, have been extensively exploited as sorbents, exchangers and pillared clays.
The 1:1 type clay minerals, the other most abundant clay mineral type, are composed of one Si-tetrahedral sheet and one Al-octahedral sheet (FIG. 2). Unlike 2:1 type minerals, the constituent sheets of all 1:1 type clays have practically no cationic substitutions. As a result, 1:1 layers are electrically neutral and lack cation exchange and swelling properties which are the centre piece of montmorillonites.
While kaolins do not possess any interlayer net negative charge, their 1:1 structure does have the advantage of having a polar interlayer region due to 1:1 sheet configuration (FIG. 2). In other words, one side of the interlayer space is lined with oxygens, while the other side is lined by hydroxyls. This polar nature of the interlayer region can potentially attract a variety of organic and inorganic compounds into the interlayer. For these reasons, kaolins possess a limited intercalation capacity while their electrically neutral 2:1 analogues, talc and pyrophyllite, exhibit no intercalation behaviour.
However, intercalation capacity of kaolins is restricted due to the layers being strongly coupled by H-bonds between the surface hydroxyls of the octahedral sheet and basal oxygens of the tetrahedral sheet. The strength of interlayer bonding and consequent intercalation capacity varies among various kaolins depending on the history of their formation. Due to the interlayer region being protected from the reacting species by strong interlayer H-bonding, any exchange and reactivity in these minerals is generally provided by OH groups on the edges of the crystals, which only make a small proportion of total OH groups present in the 1:1 minerals.
Besides this restrictive role, interlayer bonding also restricts the entry of intercalating molecules which are attracted to the polar nature of the interlayer region. Only some polar compounds such as potassium acetate (KAc) and dimethyl sulphoxide (DMSO) are able to intercalate kaolins as opposed to a wide variety that can intercalate smectites.
In the scientific press, there is abundant literature on the intercalation capacity of kaolins particularly kaolinite and halloysite. Reviews by Wada, 1959, American Mineralogist 44 153-165 and Carr et al., 1978, Clays and Clay Minerals 26 144-152 list the compounds that can intercalate kaolin clays to some degree. As is evident in these reviews, kaolins can intercalate only some highly polar molecules or salts of univalent alkali metals such as potassium acetate and alkali metal halides. Kaolins are, as these authors have stated in their reviews, unable to intercalate salts of divalent cations such as Ca, Mg, Cu, Ni etc. This is the principal reason why kaolins have not been able to find extensive applications as sorbents, catalysts and porous materials. Clearly, successful intercalation or exchange of ions or molecules of certain divalent or trivalent metals into the interlayer is necessary for preparation of useful catalytic, adsorbent or porous materials from kaolins.
Therefore, it is highly desirable to improve the intercalation capacity of the kaolin interlayer region so that desired organic or inorganic compound may be intercalated into these clays. The new intercalation complexes thus formed can be then further treated using prior art or novel processes to form new materials for applications in catalysis, ceramics, uptake of pollutants etc.
We have discovered that the ability of kaolin clays to intercalate compounds can be increased many fold. The kaolin clays modified in this way are able to readily intercalate compounds that make no or only weak complexes with untreated kaolins.
The 1:1 group clay minerals comprise the polymorphs kaolinite, nacrite, dickite and halloysite. Kaolinite, dickite and nacrite have a chemical composition Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4 and differ only in the manner in which the 1:1 layers are stacked. Halloysite has the composition Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4..sub.2 H.sub.2 O and differs from the other three polytypes in that molecular water occupies the interlayer site in its hydrated form. Kaolinite is most abundant in the kaolin group and has many applications in the paper industry, ceramics, electrical insulation and the like. This disclosure primarily describes results for kaolinite and halloysite. However, it will be appreciated by those skilled in the art that the invention applies equally to other members of the kaolin group clays.
Kaolins occur widely in many parts of the world and are, in general, more abundant than montmorillonite clays. In tropical and subtropical regions, kaolin clays are particularly common, and often overlie precious metal ores deposits. Kaolin clays also commonly underlie bauxite deposits such as bauxite deposits in Weipa, North Queensland, Australia. In such situations, kaolin clays are a cheap byproduct of metal or other ore mineral mining. Not all types of kaolins are suitable for paper coating, ceramics and other current applications of kaolins. Consequently, much of the kaolin produced as a byproduct of mining activity remains unused and poses a significant disposal problem.
Recently, there has been a renewed interest in utilising kaolin, and several new materials and processes have been disclosed which utilise kaolin clays as their raw material. For example, International Publication No. WO96/18577 discloses a process to synthesise materials of high cation exchange and moderate surface area by reacting kaolin clays with alkali metal hydroxides. A similar patent application International Publication No. WO95/00441 describes a process of treating kaolin clays with alkali metal carbonates and hydroxides at elevated temperatures to produce material of high cation exchange but low surface area. In this set of processes the kaolin structure is completely destroyed in highly basic or high temperature conditions and an X-ray amorphous material or a material of a structure completely different from layered structure of kaolin is formed.
U.S. patent (U.S. Pat. No. 5,416,051) describes a process to prepare pillared clays using kaolin clays. According to this disclosure, metakaolin is first prepared by firing untreated kaolin at temperatures sufficient to dehydroxylate kaolin and then treating the metakaolin with pillaring agents. The metakaolin prepared by calcining kaolin is a amorphous material structurally and chemically different from kaolin.
None of the disclosures in the patent literature or publications in the scientific literature attempts to enhance the intercalation capacity of kaolin clays without radically destroying its structure.
Kaolins may find additional applications if new kaolin based materials could be readily manufactured with desirable properties such as higher BET surface area and/or higher cation exchange capacity and/or higher density of active sites for applications such as catalysis. Additionally, the usefulness of these new materials made from kaolins would be greater if only one of these properties were evident or if other properties such as adsorption or uptake of other elements or compounds were enhanced over that of unprocessed kaolinite. These elements and compounds may be in the liquid or gaseous state.
Accordingly, it is an object of the present invention to provide modified forms of kaolinite which show enhanced properties over that of the original starting kaolinite through treatment with a suitable reagent.